Vacuum pumps used to reduce pressure in a process chamber, for example, pumping purge or process gas out of the chamber, can introduce undesirable heat, cooling and/or contaminants into the chamber. It is known to use thermal shielding louvers in the inlet of cryogenic pumps to reduce the cooling effect of the pump on process chambers. Due to conductance loss (undesirable distortion and/or attenuation of flow patterns through the process chamber), thermal shields are not used at the inlet of turbo-molecular pumps (TMP). The effective pumping speed of a TMP is significantly reduced by the addition of baffles or louvered thermal shields at the inlet of the pump. Further, the desired symmetry for the flow of process gas being pumped out of the chamber by the TMP would be seriously compromised by the presence of such shields. Thus, for at least the preceding reasons, thermal shields have not been implemented on TMPs.
In general, adding louvers to the inlet of high vacuum pumps significantly reduces the conductance and effective pumping speed of the pump. Hence, to attain the same results as unlouvered pumps, more louvered pumps must be used. Increasing the number of pumps increases the cost of the system and the size of the vacuum process chamber considerably. For a pump with a conductance control valve at its inlet, thermal shielding causes significant and undesirable disturbance in the flow pattern of process gas. The symmetry of gas flow is extensively disturbed and altered by the thermal shield at the pump or valve inlet through which the flow must pass. However, flow symmetry is necessary to maintain uniform local gas pressure on the component which is being processed (such as a silicon wafer or a reticle in a semiconductor fabrication or inspection tool). Thus, implementing known thermal shielding at the pump inlet, sacrifices effective pumping speed and flow control, leading to much more complex and more expensive process chambers and vacuum systems.
It is known to measure temperature and/or pressure in a process chamber; however, sensors for executing the measurements are placed on the chamber walls interfering with flow through the chamber. Further, it is known to use pressure measurements in a process chamber to correlate pump operation to pump curves for the pump. Ideally the measurements would be proximate the pump inlet; however, known pressure sensors are located on the walls of the chamber, relatively far from the pump inlet.